Genetic variation in the retroviral populations depend on the mutation and recombination rates/replication cycle, the rate of replication/time and the selective forces that exist on the population. The high mutational rate of retroviruses is due to the virally encoded reverse transcriptase, which lacks proofreading function and has low processivity. Mutations occur through misincorporation, dislocation mutagenesis, and intramolecular or intermolecular strand switching. The template switching, which is a process required for viral replication, can lead to deletions, duplications, and recombination. Previous studies has defined the mutation rates and the spectrum of mutations possible through viral replication. The current application aims at defining the mechanism that RT switches templates and the structural determinants of RT and the reverse transcription complex that are important for fidelity. Four specific aims are proposed: 1) Utilize retroviral vectors encoding directly repeated sequences to determine in vivo the relative rates of template switching in RNA and DNA- dependent DNA synthesis, and the effects of distance between direct repeats, RNA dimer linkage signal, and template-primer hydrogen bonding on template switching; 2) Perform targeted and random mutational analysis to define structural features of reverse transcriptase and RNase H that are important for accuracy of DNA synthesis. 3) Determine whether environmental factors such as nucleotide pool imbalances and anti-viral drugs affect retroviral mutation rates. 4) Probe the structure of the reverse transcription complex by determining whether DNA synthesis initiates on both co-packaged RNAs and whether minus-strand transfer is promoted by a putative circular structure of the viral RNAs.